The invention relates to a resonance trap for suppressing electromagnetic coupling phenomena for at least one line for conducting electric currents or electromagnetic waves, notably for RF alternating currents in an MR apparatus, which resonance trap includes a conductor which extends along a part of the length of the line.
In the context of a typical magnetic resonance imaging method the magnetic moment of the protons is oriented in one spatial direction by means of a strong, steady magnetic field of, for example, 1.5 Tesla. Using brief electromagnetic RF pulses, the individual protons are excited to precession and subsequently become oriented in conformity with the external, strong magnetic field again. In particular the excitation and relaxation times and the frequencies of the precessional motions are dependent on the tissue and in the context of the measurement they provide, in conjunction with a position code of the excitation, information concerning the situation in space of various tissues. The position code utilizes position-dependent frequencies and phases of the precessional excitation and enables information on the location of the relevant emission to be derived via Fourier transformation of the measured MR signal.
In order to achieve a high image quality it makes sense to utilize several characteristics so as to distinguish the types of tissue being examined. The speed at which the magnetic fields in the MR apparatus can be varied represent forms a limiting factor in achieving a high image quality within an acceptable examination time. Therefore, it is continuously being attempted to develop coil systems which enable high magnetic field strength transients to be obtained in conjunction with amplifiers and voltage sources.
In order to generate the magnetic field strength gradients desired for the position code, it is common practice to use different coil systems in three mutually perpendicular spatial directions. Two Helmholz coils which face one another are usually arranged in the direction of the longitudinal axis of the body, that is, the axis which is usually referred to as the z axis. In the direction perpendicular to the longitudinal axis of the body there are usually provided spiral-like planar coils which are arranged opposite one another so as to enclose the examination volume in a cylindrical fashion. Along the longitudinal axis of the body two or more of such spiral-like coils are often provided for a spatial direction extending perpendicularly to the longitudinal axis of the body. The two spatial directions which are oriented perpendicularly to the longitudinal axis of the body are usually referred to as the x direction and the y direction for which respective, separate coil systems of the latter kind are used.
The coils used for the precessional excitation of the protons usually are situated in the examination room enclosed by the other coils. For ease of operation the walls of the examination room are often provided with connections for the RF coils in different locations, thus enabling the coils to be arranged on the patient to be examined in conformity with the requirements of the relevant examination. The lines leading to the individual connections, however, may readily be subject to electromagnetic coupling to the RF magnetic fields of the RF coils, so that electric currents and voltages could be generated in the lines and shields. On the one hand, the voltages, currents and electromagnetic fields thus arising falsify the measuring results while on the other hand the induced voltages and currents may reach an order of magnitude such that they become a hazard for the patient to be examined.
In order to avoid at least the voltages and currents which are hazardous to the patient to be examined, it is already known to wrap a conductor around the line to the RF coils. The inductance thus formed is customarily connected in series with a capacitor which is coupled back to the line, the resonance frequency of the resonant circuit thus obtained being tuned to the frequency of the MR apparatus. The line present in the coil of the resonant circuit is thus shielded by means of the excited resonant circuit and the RF signal of the line remains unaffected. It is a drawback, however, that a stray field of the resonant circuit arises, thus necessitating the use of an additional shield for this arrangement. A further drawback in respect of the manufacture and maintenance of this device, moreover, resides in the necessary connection between the capacitor of the resonant circuit and the actual line for the RF signal. These components are connected to one another so as to be quasi inseparable, so that in the case of a defect the complete line, including the shields, must be replaced. The alternative in the form of a modular construction would necessitate a multitude of detachable connections which on the one hand would raise the manufacturing costs to an unacceptable extent while on the other hand the number of vulnerable would strongly increase to the detriment of the availability.
For the shielding of the RF conductor the U.S. Pat. No. 5,742,165 already teaches to enclose the line by means of a cylindrical conductor over a length which corresponds to one quarter of the wavelength of the electromagnetic radiation of the MR apparatus, the relevant wavelength being the wavelength present in a dielectric between the cylindrical shield and the line. At one end the cylindrical shield is short-circuited directly to the line to be shielded whereas at the other axial end it is connected thereto via a capacitor. Because of the connection via the capacitor, the electrically effective length of the cylindrical shield (also referred to as the electrical length hereinafter) is significantly reduced relative to the actual length.
This intricate arrangement has the drawback that a direct connection exists between the RF line and the individual shields, so that in the case of failure of one of the components the entire line with all shields must be replaced at high costs.
Considering the drawbacks and problems of the present state of the art, it is an object of the invention to provide a resonance trap for a line for conducting electric currents which reliably shields RF electromagnetic radiation and enables a modular construction of the resonance trap on the line.
This object is achieved in accordance with the invention by means of a resonance trap of the kind set forth in which at least one inner conductor extends along the line over a part of its length, that at least one outer conductor extends along the inner conductor, that the inner conductor is arranged at a distance from the line which is smaller than that at which the outer conductor is arranged, and that the outer conductor is arranged so as to cover the inner conductor at least partly relative to the line.
The inner conductor can then extend parallel to the line. Arrangement of the outer conductor parallel to the inner conductor ensures a structurally sensible distance between the conductors and between the conductors and the line. The line and the inner as well as the outer conductor should be situated in such a manner that their principal dimension extends at least partly along a common straight connecting line, thus ensuring high-quality shielding.
An advantage of the resonance trap in accordance with the invention resides in the fact that it is no longer necessary to connect the resonance trap conductively to the line. Because of the dissociation of the resonance trap from the lead, for the first time a modular construction can be realized in which the line and the resonance trap constitute separate components. There is no longer a need for plug-type connections between the line and the resonance trap. The construction of the line and the resonance trap as separate components at the same time enhances the robustness of these two components. The modular construction enables a variety of possibilities for standardization, so that substantial cost savings can be achieved. Finally, the resonance trap in accordance with the invention can be more readily mounted on the MR apparatus, not in the least because of its greater robustness. The line can then be simply slid through the individual resonance traps. The number of resonance traps is chosen in conformity with the length of the line.
In a further embodiment of the invention the inner conductor has an electrical length in the longitudinal direction of the line which corresponds to approximately one quarter of the wavelength in the medium between the inner conductor and the outer conductor for electromagnetic waves of the frequency of the RF alternating currents. The electrical length is dependent above all on the properties of the medium between the inner conductor and the outer conductor, notably the permittivity and the magnetic permeability. A resonance trap which is constructed with an electrical length of one quarter of the wavelength provides a suppression of electromagnetic coupling phenomena with the highest quality factor (for example, Q=250) in a comparatively narrow band region. Similar to the resonance phenomena in an electrical resonant circuit, resonance phenomena of the same kind occur in the arrangement and make the input impedance for electromagnetic waves on the line increase without a barrier in the ideal case. The inner conductor and the outer conductor are constructed so as to be approximately equally long, that is, so that their length corresponds to approximately one quarter of the wavelength which arises in the medium between the inner conductor and the outer conductor for electromagnetic waves of the frequency of the RF alternating currents of the line.
In a further embodiment of the resonance trap in accordance with the invention the inner conductor and the outer conductor are constructed as respective flat plates which extend along the line. In that case it makes sense to utilize a pair-wise oppositely situated arrangement of each time two inner and outer conductors, the line being situated between the two inner conductors. Moreover, more than two resonance traps, consisting of a planar inner conductor and a planar outer conductor, can be arranged around the line. This simple arrangement is extremely robust and achieves a surprisingly high shielding quality factor. The individual resonance traps arranged around the line need not be conductively connected to one another. The open arrangement enables flexible operation and minimizes the work required for mounting. It is also advantageous for the arrangement that the resonance trap in accordance with the invention has an adequate tolerance range with respect to the resonance frequency. A desirable shielding quality factor is obtained even in the presence of large mechanical dimensional tolerances as caused by an inexpensive manufacturing process. The dimension of the inner conductor, notably transversely of the direction of the line, may be smaller than that of the outer conductor so as to avoid undesirable coupling between the currents of the resonance trap and the environment. It is desirable to short-circuit the arrangement of the inner conductor and the outer conductor at one axial end. In order to reduce the actual length and to tune the resonance frequency, moreover, it makes sense to provide a capacitor, notably at one axial end, between the outer conductor and the inner conductor. A symmetrical combination of two resonance traps may also be advantageous. In that case a capacitor may be provided at the axial center, notably of the outer conductor, or two capacitors may interconnect the inner conductor and the outer conductor at both axial ends. In the former arrangement, that is the arrangement with the capacitor arranged at the axial center, an end short-circuited at both sides would be advantageous.
In another embodiment of the resonance trap the inner conductor and the outer conductor are constructed as respective, conductive hollow cylinders. Upon assembly the line can then be simply fed through the inner conductor, a plurality of resonance traps being arranged at distances from one another in the axial direction which amount to less than one quarter of the wavelength relative to the surrounding medium (usually the atmosphere). The particularly simple construction of the conductors as a hollow cylinder, or as a tube, enables very economical manufacture and is extremely robust. Such an arrangement is particularly suitable for the shielding of lines of small diameter. In addition to a circular or elliptical cross-section, polygonal cross-sections, for example, rectangular or octagonal cross-sections, are also feasible for the conductors of the resonance trap.
For lines of large diameter in accordance with the invention it is arranged that the inner conductor and the outer conductor are constructed as a respective conductive envelope for the line and that the two envelopes are provided with interruptions which are continuous in the longitudinal direction and are distributed along the circumference. A line of large diameter needs an enclosing resonance trap of even larger dimensions in the direction transversely of that of the conductor. As from a given magnitude, however, eddy currents occur in the conductors of comparatively large volume, said eddy currents being induced by the external gradient field. In order to avoid eddy currents which influence the gradient field, the conductors in accordance with the invention are provided with interruptions which are continuous in the longitudinal direction. When this simple step is taken, eddy currents and the associated interference fields are effectively avoided. The resonance trap can thus be constructed as a number of conductors which are distributed along the circumference and extend along the line at two different distances at least.
Different media may be present between the conductors, for example, the ambient air or an appropriate dielectric in order to reduce the actual length. It is also feasible for a liquid medium to be present between the two conductors. The medium present between the conductors may contain protons, because the formation of an interference signal is precluded by the resonance trap.
In order to form a resonance trap which is capable of resonance it is effective to connect the inner conductor conductively to the outer conductor at one axial end. The inductance of the overall arrangement constitutes a line resonator in conjunction with the capacitance of the overall arrangement.
In order to reduce the length of the resonance trap, the inner conductor can be connected, at one axial end and via a capacitor, to the outer conductor at the relevant axial end. The actual length of the resonance trap can thus be significantly reduced, so that the resonance traps which are usually to be arranged in succession in the longitudinal direction of the line are easier to handle notably during the mounting. It is to be noted that the need for shorter shields is often imposed already by the short length of the line to be shielded.
In another embodiment of the invention the two conductors are constructed so as to be symmetrical relative to their axial center, each of the two axial halves having an electrical length in the longitudinal direction of the line which corresponds to approximately one quarter of the wavelength in the medium between the inner and the outer conductor for electromagnetic waves of the frequency of the RF alternating currents of the lead. This construction of the resonance trap essentially consists of two resonance traps which are arranged axially against one another at the end of the connection between the inner conductor and the outer conductor; the currents which otherwise arise in the connection between the inner conductor and the outer conductor because of the symmetry of the arrangement now cancel one another. A conductive connection can thus be omitted between the inner conductor and the outer conductor. A corresponding construction with identical capacitors between the two outer ends of the symmetrical resonance traps reduces the actual length of the resonance trap relative to the electrical length. The electrical length of this resonance trap amounts to one half wavelength of an electromagnetic wave in the medium between the outer and the inner conductor for the frequency of the RF alternating currents of the line.
In another embodiment of the invention the inner conductor is conductively connected to the outer conductor at both actual ends, the outer conductor being interrupted by a capacitor at its axial center. The arrangement is symmetrical relative to the axial center. Because of the mirror symmetry of the arrangement, a central conductive connection between the inner and the outer conductor can be dispensed with, because the currents arising cancel one another. The electrical length of a resonance trap thus constructed amounts to half the wavelength of the frequency to be decoupled.
Another embodiment of the invention is obtained when the resonance trap is formed as part of a line envelope which can be split in the circumferential direction and has, for example, the function of a line guide or a line channel. The interruptions of the conductors which are for the suppression of eddy currents and are formed by interruptions of the resonance trap which extend in the longitudinal direction of the line can also serve as parting grooves of a line guide. The width of the grooves should be based on the requirements as regards the quality factor of the shielding, since wide grooves or wide interruptions in the conductors cause degrading of the shielding quality.
The modular construction of the resonance trap in accordance with the invention enables a line to be provided with a plurality of resonance traps which are tuned to different frequencies. These traps can be arranged, for example, sequentially or coaxially in one another. High-quality decoupling in a plurality of narrow band frequency ranges can thus be achieved for a line, thus enabling operation with different excitation frequencies and different strengths of the main field.